Electroluminescent phosphors



United States ELECTROLUMINESCENT PHOSPHORS No Drawing. ApplicationNovember 25, 1953 Serial No. 394,512

2 Claims. (Cl. 252-301.6)

This invention relates to electroluminescent phosphors, that is tophosphors which become luminescent under the action of an electricfield.

Phosphors of this type are now known, for example, properly activatedZinc sulfide. Such phosphors have previously been prepared forelectroluminescence in an oxidizing or inert atmosphere.

We have discovered, however, that the electroluminescence of suchphosphors is greatly increased if they are instead fired in a slightlyreducing atmosphere. But if the reducing nature of the atmosphere isincreased until it becomes highly reducing, the luminescence will bedecreased.

The increased luminescence with a slightly reducing atmosphere may bedue to a reduction in the number or depth of traps, because a reducingatmosphere tends to inhibit the formation of traps. The decrease inbrightness as the atmosphere is made more highly reducing may be due totoo great a reduction in the number or depth of traps, or to a reductionin the concentration of donor or acceptor atoms, which is presumablyenhanced by trap-producing conditions, since donors and acceptors, liketraps, are due to impurities or imperfections in the crystal. The effectof the reducing atmosphere may, however, be due to entirely differentreasons, for example, to some change in the surface layer of thephosphor.

Other features, objects and advantages of the invention will be apparentfrom the following specification.

In one embodiment of the invention, we mixed the following ingredientsas fine dry powders in the mole The lead can be omitted if higher coppercontents are used, say 0.002 mole per mole of zinc sulfide.

The mixture was then fired at a temperature of 1720 F. for about thirtyminutes, by being passed through a furnace held at that temperaturewhile a gaseous atmosphere flowed counter to the direction of travel ofthe mixture, for example, as described in copending application SerialNo. 230,713, filed June 8, 1951, by Keith H. Butler and Horace H. Homer,now US. Patent No. 2,728,730.

The fired powder was lightly crushed and then treated with a hotsolution of 5% acetic acid in water, then washed in a similar hot 1%acetic acid solution, and afterward in hot water.

atent O The light output was determined by mixing 1.5 grams of thephosphor with 1.25 cc. of castor oil and placing the mixture between asheet of conducting glass and a piece of metal fixed to give a spacingof 0.008 inch between the glass and the metal. A voltage of 500 volts,60 cycles per second alternating current, was applied between the pieceof conducting glass and the metal and the light measured with aphotoelectric cell and a suitable meter.

The luminescent output was as follows, varying with the atmosphereflowing over the phosphor during firing as shown:

Light output Atmosphere: relative units nitrogen, 10% hydrogen 540 v 97%nitrogen, 3% hydrogen 1035 100% nitrogen 790 79% nitrogen, 21% oxygen660 The electroluminescence of the above phosphor was largely in theblue. A green electroluminescent phosphor can be prepared in the samemanner, starting with the following mixture:

Substance: Moles ZnS (including 2% ZnCI 0.95 ZnO 0.05 PbCO 0.001 CuO0.001

The luminescent output of the resultant phosphor was as follows,depending on the atmosphere used:

Light output It is seen from the above that the slightly reducingatmosphere is best in each case. Such an atmosphere is also beneficialto phosphors containing manganese in activating amounts. For example,about 0.01 gramatoms manganese per mole of zinc sulfide, added to themixture of the second example above, before firing will give ayellow-electroluminescing phosphor, which is improved by the use ofslightly reducing atmosphere similar to the above examples.

What we claim is:

1. The method of making an electroluminescent phosphor which comprisesmixing zinc sulfide with a copper activating compound, and firing theresultant mixture in an atmosphere comprising 3% hydrogen and 97%nitrogen.

2. The method of making an electroluminescent phosphor which comprisesmixing zinc sulfide containing activating compounds of copper and lead,and firing the resultant mixture in an atmosphere of about 3% hydrogenand about 97% nitrogen.

Froelich H, article in J. Electrochem. Soc., vol. 100, No. 11, November1953, pp. 496-607.

1. THE METHOD OF MAKING AN ELECTROLUMINSCENT PHOSPHOR WHICH COMPRISESMIXING ZINC SULFIDE WITH A COPPER ACTIVATING COMPOUND, AND FIRING THERESULTANT MIXTURE IN AN ATMOSPHERE COMPRISING 3% HYDROGEN AND 97%NITROGEN.